Conventionally, there is an electrophotographic image forming apparatus. Generally this image forming apparatus electrifies a photosensitive drum uniformly to initialize the photosensitive drum, and forms an electrostatic latent image on the photosensitive drum by optical writing. The image forming apparatus transforms the electrostatic latent image to a toner image, which is in turn transferred onto a transfer material, such as a sheet of paper, directly or indirectly, and is fixed by a fixing unit.
In the system of indirectly transferring an image on a transfer member, first, primary transfer of a toner image on the circumferential surface of the photosensitive drum onto an intermediate transfer belt unit is carried out, and then secondary transfer of the toner image onto a sheet of paper from the intermediate transfer belt unit is carried out. The systems of primary transfer onto the intermediate transfer belt unit include a direct pressure type and a shift type.
FIG. 7A is a diagram schematically and exemplarily illustrating the structural relation among the conventional direct pressure type primary transfer roller, the intermediate transfer belt and the photosensitive drum. FIG. 7B is a diagram schematically and exemplarily illustrating the structural relation among the conventional shift type primary transfer roller, the intermediate transfer belt and the photosensitive drum.
While a photosensitive drum 7, an intermediate transfer belt 14 and a primary transfer roller 18 shown in FIGS. 7A and 7B have conventional structures, they are given the same reference numerals as those of the components of an image forming apparatus according to an embodiment of the invention to be described later which have similar functions.
According to the direct pressure type configuration shown in FIG. 7A, a nip width a formed with respect to the intermediate transfer belt 14 by the photosensitive drum 7 and primary transfer roller 18 is narrow, and a transfer pressure b is high, whereas according to the shift type configuration shown in FIG. 7B, a nip width c is wide, and a transfer pressure d is low.
A positional deviation (shift) amount f from the position where the photosensitive drum 7 and intermediate transfer belt 14 in FIG. 7B contact each other to the top surface of the primary transfer roller 18 is the value that forms a coefficient called “wind-around amount” in a certain calculation equation at the time of checking the transfer coefficient or the like from the other side.
The concept of “wind-around amount” comes from the fact that the state where the nip width c in the shift type becomes wider than the nip width a in the direct type is called “winding of the intermediate transfer belt around the photosensitive drum” in the field of design.
Since the shift amount f which is provided when the primary transfer roller 18 is shifted from a non-transfer position 18b to a transfer executing position 18a is an important factor that affects the action of a transfer voltage to be applied to the intermediate transfer belt 14 from the primary transfer roller 18, the shift amount f should be set accurately.
This shift amount f important to the primary transfer depends on a vertical movement amount g of the primary transfer roller 18. When the vertical movement amount g of the primary transfer roller 18 changes, therefore, the shift amount f changes, influencing the transfer state. In other words, unless the vertical movement amount g is accurately set, the adequate transfer cannot be achieved.
To accurately make the vertical movement by the vertical movement amount g to accurately set the shift amount f, it is necessary to considerably improve the accuracy of a device or structure that supports a shaft of the primary transfer roller 18. If this accuracy drops or the vertical movement amount g or the shift amount f changes during transfer, the transfer state changes, causing deficiency, such as spots, on the transferred image.
The image forming apparatuses proposed in Unexamined Japanese Patent Application KOKAI Publication Nos. H09-152791, 2001-296760 and 2007-310024 are configured to have the primary transfer roller disposed downstream of the photosensitive drum.
Although each of the publications describes positioning of the primary transfer roller at the time of transfer, however, it does not give a clear description on the setting of the shift amount f, leaving unattended the issue on how to keep the shift amount f constant.